Wrist-detection algorithm

ABSTRACT

Algorithms for detecting whether a device is properly secured to a user&#39;s skin are described. The operation of a device, such as a wearable device, can be adjusted based on whether the device is properly secured to a user&#39;s skin (e.g., on-wrist) or not properly secured to the user&#39;s skin (e.g., off-wrist). For example, certain functions can be disabled for power-saving, security or other purposes if the device is off-wrist. In order to avoid falsely identifying the device as off-wrist or on-wrist, algorithms for detecting whether the device is on-wrist or off-wrist can calculate one or more variances based on signals measured by a light sensor and compare the one or more variances with one or more thresholds. Comparing the one or more variances to the one or more threshold can improve the accuracy of wrist-detection algorithms.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/845,156 (now U.S. Publication No. 2016-0341600), filed on Sep. 3,2015, which claims benefit of U.S. Provisional Patent Application No.62/163,304, filed May 18, 2015, which is hereby incorporated byreference in their entirety.

FIELD OF THE DISCLOSURE

This relates generally to algorithms for detecting whether a device isproperly secured to a user's skin, and more specifically to usingvariance checks to determine whether the device is properly secured to auser's skin.

BACKGROUND OF THE DISCLOSURE

A device, such as a wearable device, can include one or more lightemitters and one or more light sensors. The one or more light emittersand one or more light sensors can be used to detect signals. Forexample, a photoplethysmogram (PPG) signal can be obtained by measuringthe perfusion of blood within the skin of a user. The signals detectedby the one or more light sensors can be used to detect whether thedevice is properly secured to a user's skin.

BRIEF SUMMARY OF THE DISCLOSURE

This relates to algorithms for detecting whether a device is properlysecured to a user's skin. The operation of a device, such as a wearabledevice, can be adjusted based on whether the device is properly securedto a user's skin (e.g., on-wrist) or not properly secured to the user'sskin (e.g., off-wrist). For example, certain functions can be disabledfor power-saving, security or other purposes if the device is off-wrist.In order to avoid falsely identifying the device as off-wrist oron-wrist, algorithms for detecting whether the device is on-wrist oroff-wrist can calculate one or more variances based on signals measuredby a light sensor and compare the one or more variances with one or morethresholds. Comparing the one or more variances to the one or morethresholds can improve the accuracy of wrist-detection algorithms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an example system implementing awrist-detection algorithm according to examples of the disclosure.

FIG. 2 illustrates an example high-level algorithm for performing wristdetection according to examples of the disclosure.

FIG. 3 illustrates an example high-level algorithm for performing anon-wrist check according to examples of the disclosure.

FIG. 4 illustrates an algorithm for performing variance checks accordingto examples of the disclosure.

FIG. 5 illustrates another algorithm for performing variance checksaccording to examples of the disclosure.

FIG. 6 illustrates another algorithm for performing variance checksaccording to examples of the disclosure.

FIGS. 7-9B illustrate example variance threshold levels according toexamples of the disclosure.

FIG. 10 illustrates an example high-level algorithm for performing anoff-wrist check according to examples of the disclosure.

FIG. 11 illustrates a block diagram of functional units that can becontained within or controlled by the processor to perform a variancecheck algorithm according to examples of the disclosure.

FIG. 12 illustrates a block diagram of an exemplary system architecturethat can implement the variance check algorithms according to examplesof the disclosure.

FIGS. 13A-13C illustrate systems in which examples of the disclosure canbe implemented.

DETAILED DESCRIPTION OF THE DISCLOSURE

In the following description of examples, reference is made to theaccompanying drawings which form a part hereof, and in which it is shownby way of illustration specific examples that can be practiced. It is tobe understood that other examples can be used and structural changes canbe made without departing from the scope of the disclosed examples.

This relates to algorithms for detecting whether a device is properlysecured to a user's skin. The operation of a device, such as a wearabledevice, can be adjusted based on whether the device is properly securedto a user's skin (e.g., on-wrist) or not properly secured to the user'sskin (e.g., off-wrist). For example, certain functions can be disabledfor power-saving, security or other purposes if the device is off-wrist.In order to avoid falsely identifying the device as off-wrist oron-wrist, algorithms for detecting whether the device is on-wrist oroff-wrist can calculate one or more variances based on signals measuredby a light sensor and compare the one or more variances with one or morethresholds. Comparing the one or more variances to the one or morethresholds can improve the accuracy of wrist-detection algorithms.

FIG. 1 illustrates a block diagram of an example system implementing awrist-detection algorithm according to examples of the disclosure. Asillustrated in FIG. 1, the block diagram can include a light emitter102, light sensor 104, analog-to-digital converters (ADCs) 105 a and 105b, accelerometer 106, processor 108 and input/output (I/O) unit 110.Although only a single processor 108 is shown, device 112 can includemore than one processor or other processing circuitry. These componentscan be incorporated within a physical device 112 that can be worn orheld by a user so as to secure the device to a user's skin (e.g., auser's wrist) or otherwise attached to an article of clothing worn bythe user, with the light emitter 102 and light sensor 104 positionedproximate to a user's skin. Alternately, the device 112 can be entirelyor partially incorporated within a smartphone or other portable devicesuch that a user can hold the smartphone in a manner to cause thebelow-described light beam to be reflected from the user's skin backinto a light sensor positioned within the smartphone itself. A portionof the light from light emitter 102 can be absorbed by the skin,vasculature, and/or blood, among other possibilities, and a portion canbe reflected back to a light sensor 104 co-located with the lightemitter. The signals from the light sensor 104 can include heart ratesignals due to the blood pulse wave.

Although illustrated in FIG. 1 as having only a single channel formed bya single light emitter 102 and light sensor 104, in other examplesmultiple channels can be used in the system. The multiple channels canbe created by increasing the number of emitter/sensor pairs, where eachemitter/sensor pair can create a new channel, for example. In otherexamples, multiple channels can be created using different light pathsfrom one emitter to multiple sensors (e.g., one emitter and five sensorscan produce five light paths). In yet other examples, multiple channelscan be created using different light paths from multiple emitters tomultiple sensors (e.g., two emitters and two sensors can produce fourlight paths including two paths from a first emitter to each of the twosensors and two paths from a second emitter to each of the two sensors).The one or more light emitters can produce light in ranges correspondingto infrared (IR), green, amber, blue and/or red light, among otherpossibilities. In some examples, a light emitter can be a light emittingdiode (LED) and a light sensor can be a photodiode.

The accelerometer 106 can provide acceleration output signals indicativeof acceleration due to movements of the user. For example, the device112 can be worn on a user's wrist, and the accelerometer output signalscan be indicative of the arm movements (e.g., arm swings, rotations,etc.) made by the user. In some examples, the accelerometer can be athree-axis accelerometer providing three-dimensional accelerationoutputs (e.g., three channels of acceleration outputs). Additionally,device 112 can include a gyroscope (not shown) that can provide outputsignals indicative of the orientation of the device to the processor.The output signals from the accelerometer or gyroscope can also be usedas inputs to wrist-detection algorithms.

In operation, the light emitter 102 can transmit a light beam to theuser's skin 114, and the light beam can be reflected by the user's skin114 and received by the light sensor 104. The light sensor 104 canconvert this light into an electrical signal indicative of the intensitythereof. This electrical signal can be in analog form and can beconverted into digital form by ADC 105 b. The digital signal from theADC 105 b can be fed to the processor 108. Alternatively, the digitalsignal can be stored in a memory or a buffer external to processor 108.The outputs of the accelerometer 106 can also be converted to digitalform using ADC 105 a and either fed to processor 108 or alternativelystored in a memory or a buffer external to processor 108. The processor108 can receive the digital signals from the light sensor 104 and thedigital signals from the accelerometer 106, and can process thesesignals to determine whether device 112 is properly secured to a user'swrist (“on-wrist”) or not properly secured to a user's wrist(“off-wrist”).

The I/O unit 110 can take the form of one or more of a storage device, avisual display, an audible annunciator, a touch screen integrated withdevice 112, or other output indicator. The I/O unit 110 can, underprogram control from the processor 108, provide, for example, historicalinformation in visual (e.g., numeric, tabular, graphic) or audible(e.g., synthesized voice or tone) form of a detected heart rate over aperiod of time. The I/O unit 110 can also provide, under control of theprocessor 108, average heart rate information or statistical informationof the heart rate over a prior time period or periods. As a furtherexample, the I/O unit 110 can provide current heart rate values as “realtime” or instantaneous heart rate values displayed to the userperiodically (e.g., every second) during the course of an ongoingexercise program.

The I/O unit 110 can be coupled to one or more of remote unit 118, touchscreen 120 and microphone/speaker 122 or other device via wired orwireless communication links 124. The remote unit 118 can be a smartphone or other I/O device conveniently carried or worn by the user, orcan be a distant computer or data server such as the user's homecomputer or the user's cloud storage service. The I/O unit 110 canreceive input from the remote unit 118 or can receive input from theuser by means of the touch screen 120 and/or the microphone/speaker 122.For example, I/O unit 110 can receive notifications from a remote unit118 (e.g., a smartphone) that can be displayed on touch screen 120 whenthe device 112 is determined by processor 108 to be on-wrist. I/O unit110 can also route audio information for phone calls between the remoteunit 118 and the microphone/speaker 122 of device 112.

Detecting whether device 112 is properly secured to a user's wrist ornot can be useful for optimizing operation of device 112. For example,applications requiring the device 112 be properly secured on a user'swrist can be disabled to save power when the device is determined to beoff-wrist. Additionally or alternatively, when the device is off-wrist,the data from a sensor (e.g., light sensor 104) can be assumed to becompromised (i.e., bad or invalid) data when the device is determined tobe off-wrist. For example, many health or fitness applications can bedisabled when the device 112 is off-wrist, and/or data collected bysensors, such as heart rate data, that can be corrupted when the deviceis not properly secured to a user's body can be ignored or discarded.Likewise, when the device is off-wrist, one or more components of device112 can be powered down (e.g., one or more processors can be powereddown) for additional power savings. Additionally, when the device 112 isdetermined to be off-wrist, the device 112 can indicate to a remotedevice 118 not to send notifications or calls to the device 112 (and/ordevice 112 can be configured to not receive or operate in response tosaid notifications or calls). Similarly, determining that the device 112is off-wrist can cause the device 112 to require a password to preventaccess to the device 112 as a security feature.

FIG. 2 illustrates an example high-level algorithm for performing wristdetection according to examples of the disclosure. The system (e.g.,processor 108) can detect that the touch screen 120 of device 112 hasbeen put to sleep or that a request from a remote unit (e.g., asmartphone) to remotely unlock device 112 has been received (200). Inresponse to detecting that the touch screen has been put to sleep orthat a remote unlock request has been received, the system can check theon-wrist/off-wrist state of the device (202). For example, the systemcan include a flag indicative of the state of the device, and the flagcan either be set when the device is determined to be on-wrist orcleared when the device is determined to be off-wrist. When the systempreviously determined that the device was off-wrist (e.g., on-wrist flagcleared), the system can calibrate the light sensor 104 (e.g., an IR LEDand photodetector pair) for proper operation (204). The calibration ofthe light sensor will be described in more detail below. Aftercalibration, the system can perform a first on-wrist check, described inmore detail below, to determine if the device is on-wrist (206). If thefirst on-wrist check fails, indicative that the device is off-wrist, theflag can remain not set (208). If the first on-wrist check passes,indicative that the device is now on-wrist, the system can set the flagand report that the device is on-wrist (210). When the on-wrist flag isset at 210, the system can perform a second on-wrist check (212). Thesecond on-wrist check can be the same as the first on-wrist check or canbe different than the first on-wrist check. The second on-wrist checkwill be described in more detail below. If the second on-wrist checkfails, indicative that the device is off-wrist, the system can clear theflag and report that the device is off-wrist (208). If the secondon-wrist check passes, the flag remains set and the system can performan off-wrist check (214). The off-wrist check will be described in moredetail below. If the off-wrist check fails, the system can clear theflag and report that the device is off-wrist (208). If the off-wristcheck passes, the system continues to perform the off-wrist check (214)until the off-wrist check fails.

When the system previously determined that the device was on-wrist(e.g., on-wrist flag set at 202) after detecting that the touch screenhas been put to sleep or that a remote unlock request has been received,the system can keep the flag set. The system can then proceed to theoff-wrist check at 214 without requiring the first on-wrist check or thesecond on-wrist checks.

It should be understood that representing the state of the device with aflag is only an example, and the state of the device can be representedin other forms. Additionally or alternatively, in some examples, thestate of the flag can be reported to a processor or remote device via aninterrupt signal. In some examples, the interrupt signal can be sentonly when the state of the device changes (e.g., from off-wrist toon-wrist or from on-wrist to off-wrist). In other examples, however, thestate of the device can be reported via an interrupt each time thatstate is confirmed, even when the state does not change.

FIG. 3 illustrates an example high-level algorithm for performing anon-wrist check according to examples of the disclosure. The system canacquire signal samples from a sensor (302). In some examples, the sensorcan be a photodetector. The photodetector can detect an intensity oflight received when one or more light emitters (e.g., LEDs) are emittinglight and can also detect the intensity of light received when the oneor more light emitters are not emitting light. The signal associatedwith the measurement at the photodetector when the light emitter isactive and emitting light can be represented as LED_(ON), and the signalassociated with the measurement at the photodetector when the lightemitter is inactive and not emitting light can be represented asLED_(OFF). The signal samples acquired at 302 can be the differencebetween the two measurements, LED_(ON)-LED_(OFF), or alternatively caninclude LED_(ON) and LED_(OFF) separately. The system can perform directcurrent (DC) checks on the signal samples (304). The DC checks caninclude detecting that the signal sample meets or exceeds a threshold DCsignal level. The DC checks can also include analyzing the DC signallevel of the single sample and the intensity of the LED to determine ifthe values are within ranges associated with the device secured to humanskin. For example, the DC checks can include calculating a ratio of theDC signal level to the intensity of the LED and determine if the ratiois within a range of values for a human. In some examples, the DC checkscan be a two-step process, first checking the DC content of the signalsample and, assuming the DC content of the signal is sufficient,analyzing the ratio of the DC content of the signal sample to the LEDintensity, though in other examples, these two checks can be in anyorder and independent of one another.

If the any of the DC checks fail (e.g., insufficient DC signal or ratiooutside of a human range), the system can determine the state of thedevice to be off-wrist (e.g., clear the on-wrist flag or keep the flagnot set) and, if applicable, report the off-wrist state (306). If the DCchecks pass, the system can perform variance checks on the signalsamples (308). The variance checks will be discussed in more detailbelow. If the variance checks fail, the system can determine the stateof the device to be off-wrist (e.g., clear the on-wrist flag or keep theflag not set) and, if applicable, report the off-wrist state (306). Ifthe variance checks pass, the system can determine the state of thedevice to be on-wrist (e.g., set the on-wrist flag or keep the flag set)and if applicable, report the on-wrist state (310).

Although not shown in the example of FIG. 3, the algorithm of FIG. 3 canalso include additional checks such as a proximity check, saturationcheck, and/or a light emitter calibration check that will be describedbelow.

Variance checks can helpful for properly identifying whether a device isproperly secured to a user's wrist. Human skin can be modeled as ascattering volume. Light from an LED entering human skin can bereflected back to a photodetector and the signal received at thephotodetector will change due to physical or physiological propertiessuch as movement of the person, blood movement in the body, etc. Whenthe device is on-wrist, the signal variance can be higher than when thedevice is off-wrist. The variance in the signal received at thephotodetector can thereby be used to determine that the device ison-wrist. A threshold signal variance can be used to determine whetheror not the signal sample variance corresponds to a device on-wrist oroff-wrist. The threshold level can be set based on basic observations orexperiments. In order to prevent falsely reporting that the device isoff-wrist when the variance drops below the threshold signal variancefor a short period of time, the system can require detectinglow-variance (i.e., below the threshold) signal samples for a thresholdperiod of time before determining and/or reporting the device to beoff-wrist.

FIG. 4 illustrates an algorithm for performing variance checks accordingto examples of the disclosure. The algorithm can begin by retrieving asignal sample (402). As discussed above, the signal sample can be ameasurement of the difference between the signals received at aphotodetector, i.e., LED_(ON)-LED_(OFF). The system can calculate avariance of the signal sample (VAR(LED_(ON)-LED_(OFF))) and compare thevariance to a threshold signal variance (404). If the variance meets orexceeds the threshold, corresponding to the device being on-wrist, thesystem can reset a counter that is keeping track of the number oflow-variance signal samples (406). In other examples, rather thanresetting the counter, the system can alternatively decrement thecounter by one or more values. After resetting the low-variance counter,the system can get the next signal sample to perform the varianceanalysis on the next signal sample (402). If the result from calculatingthe variance of the signal sample is below the threshold signalvariance, the system can increment the low-variance counter (408). Thesystem can then compare the value of the low-variance counter to acounter threshold (410). The counter threshold can be set such that thenumber of low-variance signal samples corresponds to receivinglow-variance signal samples for a threshold period of time. If the valueof the low-variance counter is below the counter threshold, the systemcan get the next signal sample to perform the variance analysis on thenext signal sample (402). If the value of the low-variance counter meetsor exceeds the counter threshold, the system can determine and/or reportthe device is off-wrist, e.g., by clearing the on-wrist flag and/orgenerating an interrupt signal (412).

Although the example illustrated in FIG. 4 shows a counter that can beincremented or reset, it should be understood that the algorithm can beimplemented differently so as not to rely on a counter. Similarly, theuse of a counter in other figures of this description is likewise not tobe interpreted to require a counter for implementation.

In order to improve the performance of the variance checks, in someexamples, a more robust algorithm can be implemented that includesadditional variance checks and variance thresholds. A more robustalgorithm can be used to reduce the number of false positives fordetecting the device going off-wrist and also for falsely detecting thedevice staying on-wrist. FIG. 5 illustrates another algorithm forperforming variance checks according to examples of the disclosure. Likethe algorithm illustrated in FIG. 4, the algorithm illustrated in FIG. 5can begin by retrieving a signal sample (i.e., LED_(ON)-LED_(OFF))(502). The system can perform a first variance calculation of the signalsample (i.e., VAR(LED_(ON)-LED_(OFF))) and compare the result of thefirst variance calculation to a first threshold signal variance (504).If the result of the first variance calculation meets or exceeds thefirst threshold, corresponding to the device being likely on-wrist, thesystem can compare the result of the first variance calculation to asecond threshold (506). The second variance threshold can be higher thanthe first variance threshold. If the result of the first variancecalculation meets or exceeds the second threshold, corresponding to avery high variance, the system can reset (or decrement by one or more) alow-variance counter that is keeping track of the number of low-variancesignal samples (508). After resetting the low-variance counter, thesystem can get the next signal sample to perform the variance analysison the next signal sample (502).

If the first variance of the signal sample is below the secondthreshold, corresponding to an intermediate signal variance (i.e.,between the first and second thresholds), the system can perform asecond variance calculation for the signal sample, and compare thesecond variance calculation to a third variance threshold (510). Thethird threshold can be lower than the first and second thresholds. Forexample, the second variance calculation can include calculating thedifference of the variance of the signal measured when the LED is on(VAR(LED_(ON))) and the variance of the signal measured when the LED isoff (VAR(LED_(OFF))). In other words, the second variance calculationcomputes VAR(LED_(ON))−VAR(LED_(OFF)). The result of the second variancecalculation can be compared with the third variance threshold. If theresult of the second variance calculation meets or exceeds the thirdvariance threshold, the system can reset (or decrement by one or more) alow-variance counter that is keeping track of the number of low-variancesignal samples (508). If the result of the second variance calculationis below the third variance threshold, the system can increment thelow-variance counter (512). Alternatively, the system can also incrementthe low-variance counter when the result of the first variancecalculation is below the first threshold (512).

After incrementing the low-variance counter, the system can then comparethe value of the low-variance counter to a counter threshold (514). Thecounter threshold can be set such that the number of low-variance signalsamples corresponds to receiving low-variance signal samples for athreshold period of time. If the value of the low-variance counter isbelow the counter threshold, the system can get the next signal sampleto perform the variance analysis on the next signal sample (502). If thevalue of the low-variance counter meets or exceeds the counterthreshold, the system can determine and/or that report the device isoff-wrist, e.g., by clearing the on-wrist flag and/or generating aninterrupt signal (516). Although the example illustrated in FIG. 5 showsa counter that can be incremented or reset, it should be understood thatthe algorithm can be implemented differently so as not to rely on acounter.

FIG. 6 illustrates another algorithm for performing variance checksaccording to examples of the disclosure. Like the algorithm illustratedin FIG. 5, the algorithm illustrated in FIG. 6 can begin by retrieving asignal sample (i.e., LED_(ON)-LED_(OFF)) (602). The system can perform afirst variance calculation of the signal sample (i.e.,VAR(LED_(ON)-LED_(OFF))) and compare the result of the first variancecalculation to a first threshold signal variance (604). If the result ofthe first variance calculation meets or exceeds the first threshold, thesystem can compare the result of the first variance calculation to asecond threshold (606). The second variance threshold can be higher thanthe first variance threshold. If the result of the first variancecalculation meets or exceeds the second threshold, corresponding to avery high variance, the system can reset (or decrement by one or more) alow-variance counter that is keeping track of the number of low-variancesignal samples (608). After resetting the low-variance counter, thesystem can get the next signal sample to perform the variance analysison the next signal sample (602). If the first variance of the signalsample is below the second threshold, the system can get the next signalsample to perform the variance analysis on the next signal samplewithout incrementing or resetting the counter (602).

If the result of the first variance calculation is below the firstthreshold, the system can compare the result of the first variancecalculation to a third threshold (610). The third threshold can be lowerthan the first and second thresholds. If the result of the firstvariance calculation meets or exceeds the third threshold, the systemcan perform a second variance calculation for the signal sample, andcompare the second variance calculation to a fourth variance threshold(612). For example, the second variance calculation can includecalculating the difference of the variance of the signal measured whenthe LED is on (VAR(LED_(ON))) and the variance of the signal measuredwhen the LED is off (VAR(LED_(OFF))). In other words, the secondvariance calculation computes VAR(LED_(ON))−VAR(LED_(OFF)). The resultof the second variance calculation can be compared with the fourthvariance threshold. The fourth variance threshold can be lower than thefirst, second, and third thresholds. If the result of the secondvariance calculation meets or exceeds the fourth variance threshold, thesystem can reset (or decrement by one or more) a low-variance counterthat is keeping track of the number of low-variance signal samples(608). If the result of the second variance calculation is below thefourth variance threshold, the system can increment the low-variancecounter (614). Alternatively, the system can also increment thelow-variance counter when the result of the first variance calculationis below the third threshold (614).

After incrementing the low-variance counter, the system can then comparethe value of the low-variance counter to a counter threshold (616). Thecounter threshold can be set such that the number of low-variance signalsamples corresponds to receiving low-variance signal samples for athreshold period of time. If the value of the low-variance counter isbelow the counter threshold, the system can get the next signal sampleto perform the variance analysis on the next signal sample (602). If thevalue of the low-variance counter meets or exceeds the counterthreshold, the system can determine and/or report that the device isoff-wrist, e.g., by clearing the on-wrist flag and/or generating aninterrupt signal (618). Although the example illustrated in FIG. 6 showsa counter that can be incremented or reset, it should be understood thatthe algorithm can be implemented differently so as not to rely on acounter.

In some examples, the first on-wrist check 206 can follow the algorithmof either FIG. 4 or FIG. 5, and the second on-wrist check 212 can followthe algorithm of FIG. 6, though in other examples, the first and secondon-wrist checks can use different algorithms. The duration of the firston-wrist check and the second on-wrist check can also be the same insome examples, though in other examples the first on-wrist check can bea shorter duration check than the second on-wrist check. Alternatively,the duration of the second on-wrist check can be shorter than the firston-wrist check. Additionally or alternatively, the variance thresholdlevels and/or counter threshold levels can be the same or differentbetween the algorithms of FIGS. 4-6.

FIGS. 7-9B illustrate example variance threshold levels according toexamples of the disclosure. For example, FIG. 7 illustrates an examplevariance threshold level that can correspond to the variance thresholdlevel of the algorithm of FIG. 4. FIG. 7 shows a plot of the variance ofthe signal samples (i.e., VAR(LED_(ON)-LED_(OFF)) versus time. Theresults of the variance calculation that meet or exceed variancethreshold 702 can correspond to an on-wrist condition (i.e., where thelow-variance counter can be reset or decremented). The results of thevariance calculation that are below the variance threshold 702 cancorrespond to an off-wrist condition (i.e., where the low-variancecounter can be incremented).

Similarly, FIG. 8A illustrates example variance threshold levels thatcan correspond to the first and second variance threshold levels of thealgorithm of FIG. 5. Like FIG. 7, FIG. 8A illustrates a plot of theresult of the first variance calculation of the signal samples (i.e.,VAR(LED_(ON)-LED_(OFF)) versus time. The results of the first variancecalculation that are below first variance threshold 802 can correspondto an off-wrist condition (i.e., where the low-variance counter can beincremented). The results of the first variance calculation that meet orexceed both the first variance threshold 802 and the second variancethreshold 804 can correspond to an on-wrist condition (i.e., where thelow-variance counter can be reset or decremented). The results of thefirst variance calculation that meet or exceed the first variancethreshold 802 but are below the second variance threshold 804 cancorrespond to signal samples that require the second variancecalculation and threshold comparison described above with respect toFIG. 5 before making a determination about the condition of the device.

FIG. 8B illustrates a plot of the result of the second variancecalculation of the signal samples (i.e., VAR(LED_(ON))−VAR(LED_(OFF)))versus time. The results of the second variance calculation that meet orexceed the third variance threshold 806 can correspond to an on-wristcondition (i.e., where the low-variance counter can be reset ordecremented). The results of the variance calculation that are below thethird variance threshold 806 can correspond to an off-wrist condition(i.e., where the low-variance counter can be incremented).

Likewise, FIG. 9A illustrates example variance threshold levels that cancorrespond to the first, second, and third variance threshold levels ofthe algorithm of FIG. 6. Like FIGS. 7 and 8A, FIG. 9A illustrates a plotof the result of the first variance calculation of the signal samples(i.e., VAR(LED_(ON)-LED_(OFF)) versus time. The results of the firstvariance calculation that are below a first variance threshold 902 and athird variance threshold 904 can correspond to an off-wrist condition(i.e., where the low-variance counter can be incremented). The resultsof the first variance calculation that meet or exceed both the firstvariance threshold 902 and a second variance threshold 906 cancorrespond to an on-wrist condition (i.e., where the low-variancecounter can be reset or decremented). The results of the first variancecalculation that meet or exceed the first variance threshold 902 but arebelow the second variance threshold 906 can correspond to a conditionwhere the counter is not incremented, decremented or reset. The resultsof the first variance calculation that are below the first variancethreshold 902 but meet or exceed the third variance threshold 904 cancorrespond to signal samples that require the second variancecalculation and threshold comparison described above with respect toFIG. 6 before making a determination about the condition of the device.

FIG. 9B illustrates a plot of the result of the second variancecalculation of the signal samples (i.e., VAR(LED_(ON))−VAR(LED_(OFF)))versus time. The results of the second variance calculation that meet orexceed the fourth variance threshold 908 can correspond to an on-wristcondition (i.e., where the low-variance counter can be reset ordecremented). The results of the variance calculation that are below thefourth variance threshold 908 can correspond to an off-wrist condition(i.e., where the low-variance counter can be incremented).

Returning back to the algorithm of FIG. 2, the off-wrist check 214 can,in some examples, be implemented using the same variance checkalgorithms as the second on-wrist check 212. In some examples, theoff-wrist check 214 can use different variance thresholds than used forthe second on-wrist check 212. For example, the variance thresholds canbe lowered for the off-wrist check so the device is less likely tofalsely report an off-wrist condition.

FIG. 10 illustrates an example high-level algorithm for performing anoff-wrist check according to examples of the disclosure. The system canreceive one or more signal samples, such as LED_(ON)-LED_(OFF), forexample (1002). The system can first perform a saturation check (1004)to determine whether the sensor (e.g., photodetector) is saturated bylight from the ambient conditions and/or the light emitter (e.g., LED).The saturation check can, for example, first check for completesaturation due to ambient light. For example, if the signal levelreceived by the photodetector is at 100% of the signal level permittedby the hardware when the LED is off (absolute saturation), the check canfail when the saturation persists for a first threshold period of time(e.g., 1-3 seconds). Such a case can correspond to ambient conditionsthat saturate the photodetector whether or not the LED is on, which canresult in LED_(ON)-LED_(OFF)=0. When the saturation check fails underabsolute saturation conditions, the system can determine and/or reportthat the device is off-wrist (1010). If the photodetector is notsaturated when the LED is off, but is saturated when the LED is on, thesaturation check will fail, but the result of the failed saturation testcan depend on the degree of saturation of the photodetector when the LEDis off. For example, when the photodetector is saturated more than athreshold amount of the photodetector signal range for a secondthreshold period of time, the saturation check can fail and the systemcan determine and/or report that the device is off-wrist (1010). Thethreshold amount can be 60%, for example, and more generally can be setin a range of 50-100% of the photodetector signal range. Alternatively,if the photodetector is not saturated more than the threshold amount forthe second threshold period of time, the saturation test can still fail(because the photodetector is saturated when the LED is on), but thesystem can perform a recalibration of the LED and photodetector, andthen rerun the saturation checks.

Recalibrating the LED and photodetector can include adjusting theintensity of the LED output such that the signal received by thephotodetector is less than 50% of the photodetector signal range. Insome examples, the intensity of the LED can be adjusted so that thereceived signal at the photodetector is between 20% and 50% of thephotodetector signal range.

If the saturation check passes, the system can perform a calibrationcheck for the light emitter and sensor (1006). The calibration checkcan, for example, check the signal level of the photodetector when theLED is on to determine if it is within a specific range. For example,the calibration check can determine whether the signal detected by thephotodetector when the LED is on is less than 50% of the range of thephotodetector. In some examples, the calibration test can determinewhether the signal detected by the photodetector is between 20% and 50%of the photodetector signal range. In other examples, the calibrationtest can determine whether the signal detected by the photodetector isbetween 25% and 45% of the photodetector signal range. If thecalibration check passes, the system can perform proximity checks(1008). If the calibration checks fail, the system can re-calibrate theLED as described above. If the re-calibration fails, the system candetermine and/or report that the device is off-wrist (1010).Alternatively, when the calibration checks fail after recalibration(i.e., when recalibration occurs in response to a failed saturationcheck) has been performed, the system can determine and/or report thatthe device is off-wrist (1010).

The proximity checks can look at the signal sample and detect whetherthere is a drop in signal level of a threshold amount for a thresholdperiod of time. For example, a drop in the signal level by more than 25%for 5 seconds can be indicative of a change in proximity between theuser's skin and the photodetector and can correspond to a removal of thedevice from the skin (i.e., a removal event). In other examples thethreshold amount of signal level can be set between 15% and 50% of thepeak or the running average of recent signal measurements. Additionally,the threshold period of time can be set between 1 and 10 seconds, insome examples. If the proximity check fails (i.e., detects a removalevent), the system can determine and/or report that the device isoff-wrist (1010). If the proximity checks pass, the system can performDC checks (1012).

The DC checks can be the same as the DC checks described above. If theDC checks fail, the system can determine and/or report that the deviceis off-wrist (1010). If the DC checks pass, the system can performvariance checks (1014). The variance checks can be the same as thevariance checks described above, though the variance threshold can bedifferent than the variance checks for on-wrist checks. If the variancechecks fail, the system can determine and/or report that the device isoff-wrist (1010). If the variance checks pass, the system can get thenext sample(s) for analysis (1002).

FIG. 11 illustrates a block diagram of functional units that can becontained within or controlled by the processor to perform a variancecheck algorithm according to examples of the disclosure. The functionalunits can be implemented as discrete hardware units such as, forexample, digital signal processors (DSP), application specificintegrated circuits (ASIC), field programmable logic arrays (FPGA), orthe like. The functional units can be combined into one or moreinterconnected devices. Alternatively, the functional units can beimplemented in the form of software or firmware (or a combination of thetwo) configured to operate a programmable processor. Further, thefunctional units can be a combination of discrete hardware, software andfirmware.

Block diagram 1100 includes sensor 1102 for measuring signals that canbe used to determine the on-wrist/off-wrist state of the device. Forexample, the sensor 1102 can include one or more light emitter and lightsensor pairs, for example an LED and photodetector pair. The sensor cansample the photodetector at regular intervals, e.g. 5-20 Hz or 50-200ms. The sensor can also take two samples during each sampling period.One sample can be taken when the LED is on (LED_(ON)) and one sample canbe taken when the LED is off (LED_(OFF)). The LED_(ON) and LED_(OFF)samples can be taken proximate to one another, e.g., within 250microseconds. Reducing the time between samples can result in bettercorrelation of the samples.

Block diagram 1100 can also include a buffer unit 1104 to store and/orpipeline the LED_(ON) and LED_(OFF) signals received from the bufferunit 1104. In some examples, the buffer unit can separately buffer theLED_(ON) signals and LED_(OFF) signals. Buffer unit 1104 can be one ormore first-in first-out (FIFO) buffers configured to receive signalssampled from sensor 1102. Buffer unit 1104 can supply the appropriatequantity of LED_(ON) and LED_(OFF) signals for additional processing. Insome examples, the buffer unit 1104 can be controlled by a timing andcontrol unit (not shown).

Block diagram 1100 can also include a first subtraction unit 1106 and afirst variance unit 1108. The first subtraction unit 1106 and the firstvariance unit 1108 can receive the LED_(ON) and LED_(OFF) signals andcalculate the variance of the difference between LED_(ON) and LED_(OFF).The output of the first subtraction unit 1106 can be represented by theexpression LED_(ON)-LED_(OFF), and the output of the first variance unit1108 can be represented by the expression VAR(LED_(ON)-LED_(OFF)). Blockdiagram 1100 can also include a second variance unit 1114 and a secondsubtraction unit 1116. The second subtraction unit 1116 and the secondvariance unit 1114 can receive the LED_(ON) and LED_(OFF) signals andcalculate the difference between the variance of the LED_(ON) signalsand the variance of the LED_(OFF) signals. The output of the secondvariance unit 1114 can be represented by the expressions VAR(LED_(ON))and VAR(LED_(OFF)), and the output of the second subtraction unit 1116can be represented by the expression VAR(LED_(ON))−VAR(LED_(OFF)).

Block diagram 1100 can also include a comparison unit 1110. Thecomparison unit 1110 can receive output from the first variance unit1108 and from the second subtraction unit 1116 and can receive one ormore thresholds. The comparison unit 1110 can compare the output fromthe first variance unit 1108 (i.e., the first variance calculation,VAR(LED_(ON)-LED_(OFF))) and the output from the second subtraction unit1116 (i.e., the second variance calculation,VAR(LED_(ON))−VAR(LED_(OFF))) to the one or more thresholds according tothe algorithms described above. Block diagram 1100 can also include thecounter unit 1112 that can be incremented, decremented, and/or resetbased on comparisons of the first and/or second variance calculations tothe one or more thresholds. Comparison unit 1110 can also compare thevalue of the counter unit 1112 to a counter threshold to determinewhether the device is on-wrist or off-wrist.

A system architecture implementing the variance check algorithms can beincluded in any portable or non-portable device including but notlimited to a wearable device (e.g., smart band, health band, smartwatch), a communication device (e.g., mobile phone, smart phone), amulti-media device (e.g., MP3 player, TV, radio), a portable or handheldcomputer (e.g., tablet, netbook, laptop), a desktop computer, anAll-In-One desktop, a peripheral device, or any other system or deviceadaptable to the inclusion of the system architecture, includingcombinations of two or more of these types of devices. FIG. 12illustrates a block diagram of an exemplary system architecture 1200that can implement the variance check algorithms according to examplesof the disclosure. System architecture 1200 can generally include one ormore computer-readable media 1201, processing system 1204, I/O subsystem1206, radio frequency (RF) circuitry 1208, audio circuitry 1210, andsensors circuitry 1211. These components can be coupled by one or morecommunication buses or signal lines 1203.

It should be understood that the exemplary architecture shown in FIG. 12can have more or fewer components than shown, or a differentconfiguration of components. The various components shown in FIG. 12 canbe implemented in hardware, software, firmware or any combinationthereof, including one or more signal processing and/or applicationspecific integrated circuits.

RF circuitry 1208 can be used to send and receive information over awireless link or network to one or more other devices and includeswell-known circuitry for performing this function. RF circuitry 1208 andaudio circuitry 1210 can be coupled to processing system 1204 viaperipherals interface 1216. Peripherals interface 1216 can includevarious known components for establishing and maintaining communicationbetween peripherals and processing system 1204. Audio circuitry 1210 canbe coupled to audio speaker 1250 and microphone 1252 and can includeknown circuitry for processing voice signals received from peripheralsinterface 1216 to enable a user to communicate in real-time with otherusers. In some examples, audio circuitry 1210 can include a headphonejack (not shown). Sensors circuitry 1211 can be coupled to varioussensors including, but not limited to, one or more light emitting diodes(LEDs) or other light emitters, one or more photodiodes or other lightsensors, one or more photothermal sensors, a magnetometer, anaccelerometer, a gyroscope, a barometer, a compass, a proximity sensor,a camera, an ambient light sensor, a thermometer, a global positioningsystem (GPS) sensor, and various system sensors which can senseremaining battery life, power consumption, processor speed, CPU load,and the like.

Peripherals interface 1216 can couple the input and output peripheralsof the system 1200 to one or more processors 1218 and one or morecomputer-readable media 1201 via a controller 1220. The one or moreprocessors 1218 communicate with the one or more computer readable media1201 via the controller 1220. The one more computer-readable media 1201can be any device or medium that can store code and/or data for use bythe one or more processors 1218. In some examples, medium 1201 can be anon-transitory computer-readable storage medium. Medium 1201 can includea memory hierarchy, including but not limited to cache, main memory andsecondary memory. The memory hierarchy can, as non-limiting examples, beimplemented using any combination of RAM (e.g., SRAM, DRAM, SDRAM), ROM,FLASH, magnetic and/or optical storage devices, such as disk drives,magnetic tape, compact disks (CDs) and digital video discs (DVDs).Medium 1201 can also include a transmission medium for carryinginformation bearing signals indicative of computer instructions or data(with or without a carrier wave upon which the signals can bemodulated). For example, the transmission medium can include acommunications network, including but not limited to the Internet (alsoreferred to as the World Wide Web), intranet(s), Local Area Networks(LANs), Wide Local Area Networks (WLANs), Storage Area Networks (SANs),Metropolitan Area Networks (MAN) and the like.

One or more processors 1218 can run various software components storedin medium 1201 to perform various functions for system architecture1200. In some examples, the software components can include operatingsystem 1222, communication module (or set of instructions) 1224, touchprocessing module (or set of instructions) 1226, graphics module (or setof instructions) 1228, and one or more applications (or set ofinstructions) 1223. Each of these modules and above noted applicationscan correspond to a set of instructions for performing one or morefunctions described above and the methods described in this application(e.g., the computer-implemented methods and other information processingmethods described herein). These modules (i.e., sets of instructions)need not be implemented as separate software programs, procedures ormodules, and thus various subsets of these modules can be combined orotherwise re-arranged in various examples. In some examples, medium 1201can store a subset of the modules and data structures identified above.Furthermore, medium 1201 can store additional modules and datastructures not described above.

Operating system 1222 can include various procedures, sets ofinstructions, software components and/or drivers for controlling andmanaging general system tasks (e.g., memory management, storage devicecontrol, power management, etc.) and facilitates communication betweenvarious hardware and software components.

Communication module 1224 can facilitate communication with otherdevices over one or more external ports 1236 or via RF circuitry 1208and can include various software components for handling data receivedfrom RF circuitry 1208 and/or external port 1236.

Graphics module 1228 can include various known software components forrendering, animating and displaying graphical objects on a displaysurface. In examples in which touch I/O device 1212 is a touch sensingdisplay (e.g., touch screen), graphics module 1228 can includecomponents for rendering, displaying, and animating objects on the touchsensing display. The touch I/O device 1212 and/or the other I/O device1214 can comprise the I/O unit 110 of FIG. 1, and can also incorporate aUI interface permitting a use to select among programming modes ofdisplaying heart rate data when the I/O device is incorporated into adevice 112 of FIG. 1. Further, in relation to FIG. 1, the light emitter102 and light sensor 104 can be part of the I/O device 1214, and thetouch screen 120 can correspond to the touch I/O device 1212 of FIG. 12.The I/O unit 110 either integrated within device 112 or via coupling tomicrophone/speaker 122 can also provide audio outputs as part of theuser communications corresponding to audio circuitry 1210 of FIG. 12.Microphone 1252 of FIG. 12 can correspond to the microphone/speaker 122of FIG. 1.

One or more applications 1223 can include any applications installed onsystem 1200, including without limitation, a browser, address book,contact list, email, instant messaging, word processing, keyboardemulation, widgets, JAVA-enabled applications, encryption, digitalrights management, voice recognition, voice replication, locationdetermination capability (such as that provided by the GPS), a musicplayer, etc.

Touch processing module 1226 can include various software components forperforming various tasks associated with touch I/O device 1212 includingbut not limited to receiving and processing touch input received fromtouch I/O device 1212 via touch I/O device controller 1232.

I/O subsystem 1206 can be coupled to touch I/O device 1212 and one ormore other I/O devices 1214 for controlling or performing variousfunctions. Touch I/O device 1212 can communicate with processing system1204 via touch I/O device controller 1232, which can include variouscomponents for processing user touch input (e.g., scanning hardware).One or more other input controllers 1234 can receive/send electricalsignals from/to other I/O devices 1214. Other I/O devices 1214 caninclude physical buttons, dials, slider switches, sticks, keyboards,touch pads, additional display screens, or any combination thereof.

If embodied as a touch screen, touch I/O device 1212 can display visualoutput to the user in a GUI. The visual output can include text,graphics, video, and any combination thereof. Some or all of the visualoutput can correspond to user-interface objects. Touch I/O device 1212can form a touch sensing surface that accepts touch input from the user.Touch I/O device 1212 and touch screen controller 1232 (along with anyassociated modules and/or sets of instructions in medium 1201) candetect and track touches or near touches (and any movement or release ofthe touch) on touch I/O device 1212 and can convert the detected touchinput into interaction with graphical objects, such as one or moreuser-interface objects. In the case in which touch I/O device 1212 isembodied as a touch screen, the user can directly interact withgraphical objects that can be displayed on the touch screen.Alternatively, in the case in which touch I/O device 1212 is embodied asa touch device other than a touch screen (e.g., a touch pad), the usercan indirectly interact with graphical objects that can be displayed ona separate display screen embodied as I/O device 1214.

Touch I/O device 1212 can be analogous to the multi-touch sensingsurface described in the following U.S. Pat. No. 6,323,846 (Westerman etal.), U.S. Pat. No. 6,570,557 (Westerman et al.), and/or U.S. Pat. No.6,677,932 (Westerman), and/or U.S. Patent Publication 2002/0015024A1.

In examples for which touch I/O device 1212 is a touch screen, the touchscreen can use liquid crystal display (LCD) technology, light emittingpolymer display (LPD) technology, organic LED (OLED), or organic electroluminescence (OEL), although other display technologies can be used inother examples.

Feedback can be provided by touch I/O device 1212 based on the user'stouch input as well as a state or states of what is being displayedand/or of the computing system. Feedback can be transmitted optically(e.g., light signal or displayed image), mechanically (e.g., hapticfeedback, touch feedback, force feedback, or the like), electrically(e.g., electrical stimulation), olfactory, acoustically (e.g., beep orthe like), or the like or any combination thereof and in a variable ornon-variable manner.

System architecture 1200 can also include power system 1244 for poweringthe various hardware components and can include a power managementsystem, one or more power sources, a recharging system, a power failuredetection circuit, a power converter or inverter, a power statusindicator and any other components typically associated with thegeneration, management and distribution of power in portable devices.

In some examples, peripherals interface 1216, one or more processors1218, and memory controller 1220 of the processing system 1204 can beimplemented on a single chip. In some other examples, they can beimplemented on separate chips.

FIGS. 13A-13C illustrate systems in which examples of the disclosure canbe implemented. FIG. 13A illustrates an exemplary mobile telephone 1336that can include a touch screen 1324. FIG. 13B illustrates an exemplarymedia player 1340 that can include a touch screen 1326. FIG. 13Cillustrates an exemplary wearable device 1344 that can include a touchscreen 1328 and can be attached to a user using a strap 1346. Thesystems of FIGS. 13A-13C can implement variance check algorithmsdescribed herein.

Therefore, according to the above, some examples of the disclosure aredirected to a device. The device can comprise a sensor configured togenerate signals and processing circuitry capable of calculating one ormore variances based on the generated signals and determining whetherthe device is secured or not secured to an object based on the one ormore variances. The object can be a human wrist. Some examples of thedisclosure are directed to a method executed by processing circuitry forpredicting a heart rate. The method can comprise receiving signalsgenerated by a sensor, calculating one or more variances based on thegenerated signals, and determining whether the device is secured or notsecured to an object based on the one or more variances. Some examplesof the disclosure are directed to non-transitory computer readablestorage medium. The computer readable medium can contain instructionsthat, when executed, perform a method for operating an electronicdevice. The electronic device can include a processor. The method cancomprise receiving signals generated by a sensor, calculating one ormore variances based on the generated signals, and determining whetherthe device is secured or not secured to an object based on the one ormore variances.

Some examples of the disclosure are directed to a device. The device cancomprise a sensor configured to generate signal samples and processingcircuitry. The processing circuitry can be capable of performing one ormore variance calculations for each signal sample, predicting acondition of the device based on the one or more variance calculationsfor each signal sample, and determining that the device is not securedto an object when the predictions of the condition of the device for thesignal samples meets a threshold. Additionally or alternatively to oneor more of the examples disclosed above, the sensor can comprise a lightemitter and a light detector. Additionally or alternatively to one ormore of the examples disclosed above, the light emitter can a lightemitting diode (e.g., in the infrared range). Additionally oralternatively to one or more of the examples disclosed above, eachsignal sample can include a first signal generated when the lightemitter is on and a second signal generated when the light emitter isoff. Additionally or alternatively to one or more of the examplesdisclosed above, the first signal and second signal of a signal samplecan generated within threshold length of time (e.g., 250 microseconds).Additionally or alternatively to one or more of the examples disclosedabove, the one or more variance calculations can comprise a firstvariance calculation of a variance of a difference between the firstsignal and the second signal of the signal sample. Additionally oralternatively to one or more of the examples disclosed above, predictingthe condition of the device based on the one or more variancecalculations for each signal sample can comprise, predicting a firstcondition of the device when the first variance calculation is below afirst variance threshold and predicting a second condition of the devicewhen the first variance calculation meets or exceeds the first variancethreshold. Additionally or alternatively to one or more of the examplesdisclosed above, the one or more variance calculations can comprise asecond variance calculation of a difference between a variance of thefirst signal of the signal sample and a variance of the second signal ofthe signal sample. Additionally or alternatively to one or more of theexamples disclosed above, predicting the condition of the device basedon the one or more variance calculations for each signal sample cancomprise predicting a first condition of the device when the firstvariance calculation is below a first variance threshold; predicting thefirst condition of the device when the first variance calculation meetsor exceeds the first variance threshold, the first variance calculationis below a second variance threshold, and the second variancecalculation is below a third variance threshold; predicting a secondcondition of the device when the first variance calculation meets orexceeds the first variance threshold and the first variance calculationmeets or exceeds the second variance threshold; and predicting thesecond condition of the device when the first variance calculation meetsor exceeds the first variance threshold, the first variance calculationis below the second variance threshold, and the second variancecalculation meets or exceeds the third variance threshold. Additionallyor alternatively to one or more of the examples disclosed above, thesecond variance threshold can be greater than the first variancethreshold. Additionally or alternatively to one or more of the examplesdisclosed above, the third variance threshold can be less than the firstvariance threshold. Additionally or alternatively to one or more of theexamples disclosed above, predicting the condition of the device basedon the one or more variance calculations for each signal sample cancomprise predicting a first condition of the device when the firstvariance calculation is below a first variance threshold and the firstvariance calculation is below a third variance threshold; predicting thefirst condition of the device when the first variance calculation isbelow the first variance threshold, the first variance calculation meetsor exceeds the third variance threshold, and the second variancecalculation is below a fourth variance threshold; predicting a secondcondition of the device when the first variance calculation meets orexceeds the first variance threshold and the first variance calculationmeets or exceeds the second variance threshold; and predicting thesecond condition of the device when the first variance calculation isbelow the first variance threshold, the first variance calculation meetsor exceeds the third variance threshold, and the second variancecalculation meets or exceeds the fourth variance threshold. Additionallyor alternatively to one or more of the examples disclosed above, thesecond variance threshold can be greater than the first variancethreshold. Additionally or alternatively to one or more of the examplesdisclosed above, the third variance threshold can be less than the firstvariance threshold. Additionally or alternatively to one or more of theexamples disclosed above, the fourth variance threshold can be less thanthe third variance threshold. Additionally or alternatively to one ormore of the examples disclosed above, the first condition can correspondto a prediction that the device is not secured to the object and thesecond condition can correspond to a prediction that the device issecured to the object. Additionally or alternatively to one or more ofthe examples disclosed above, determining that the device is not securedto the object when the predictions of the condition of the device forthe signal samples meets the threshold can comprise predicting the firstcondition for a threshold number of consecutive predictions.Additionally or alternatively to one or more of the examples disclosedabove, the object can be human skin. Additionally or alternatively toone or more of the examples disclosed above, the object can be a wrist.

Some examples of the disclosure are directed to a method executed byprocessing circuitry for determining that a device is not secured to anobject. The method can comprise generating a plurality of signalsamples, performing one or more variance calculations for each signalsample, predicting a condition of the device based on the one or morevariance calculations for each signal sample, and determining that thedevice is not secured to the object when the predictions of thecondition of the device for the signal samples meets a threshold.Additionally or alternatively to one or more of the examples disclosedabove, the sensor can comprise a light emitter and a light detector.Additionally or alternatively to one or more of the examples disclosedabove, the light emitter can a light emitting diode (e.g., in theinfrared range). Additionally or alternatively to one or more of theexamples disclosed above, each signal sample can include a first signalgenerated when the light emitter is on and a second signal generatedwhen the light emitter is off. Additionally or alternatively to one ormore of the examples disclosed above, the first signal and second signalof a signal sample can generated within threshold length of time (e.g.,250 microseconds). Additionally or alternatively to one or more of theexamples disclosed above, the one or more variance calculations cancomprise a first variance calculation of a variance of a differencebetween the first signal and the second signal of the signal sample.Additionally or alternatively to one or more of the examples disclosedabove, predicting the condition of the device based on the one or morevariance calculations for each signal sample can comprise, predicting afirst condition of the device when the first variance calculation isbelow a first variance threshold and predicting a second condition ofthe device when the first variance calculation meets or exceeds thefirst variance threshold. Additionally or alternatively to one or moreof the examples disclosed above, the one or more variance calculationscan comprise a second variance calculation of a difference between avariance of the first signal of the signal sample and a variance of thesecond signal of the signal sample. Additionally or alternatively to oneor more of the examples disclosed above, predicting the condition of thedevice based on the one or more variance calculations for each signalsample can comprise predicting a first condition of the device when thefirst variance calculation is below a first variance threshold;predicting the first condition of the device when the first variancecalculation meets or exceeds the first variance threshold, the firstvariance calculation is below a second variance threshold, and thesecond variance calculation is below a third variance threshold;predicting a second condition of the device when the first variancecalculation meets or exceeds the first variance threshold and the firstvariance calculation meets or exceeds the second variance threshold; andpredicting the second condition of the device when the first variancecalculation meets or exceeds the first variance threshold, the firstvariance calculation is below the second variance threshold, and thesecond variance calculation meets or exceeds the third variancethreshold. Additionally or alternatively to one or more of the examplesdisclosed above, the second variance threshold can be greater than thefirst variance threshold. Additionally or alternatively to one or moreof the examples disclosed above, the third variance threshold can beless than the first variance threshold. Additionally or alternatively toone or more of the examples disclosed above, predicting the condition ofthe device based on the one or more variance calculations for eachsignal sample can comprise predicting a first condition of the devicewhen the first variance calculation is below a first variance thresholdand the first variance calculation is below a third variance threshold;predicting the first condition of the device when the first variancecalculation is below the first variance threshold, the first variancecalculation meets or exceeds the third variance threshold, and thesecond variance calculation is below a fourth variance threshold;predicting a second condition of the device when the first variancecalculation meets or exceeds the first variance threshold and the firstvariance calculation meets or exceeds the second variance threshold; andpredicting the second condition of the device when the first variancecalculation is below the first variance threshold, the first variancecalculation meets or exceeds the third variance threshold, and thesecond variance calculation meets or exceeds the fourth variancethreshold. Additionally or alternatively to one or more of the examplesdisclosed above, the second variance threshold can be greater than thefirst variance threshold. Additionally or alternatively to one or moreof the examples disclosed above, the third variance threshold can beless than the first variance threshold. Additionally or alternatively toone or more of the examples disclosed above, the fourth variancethreshold can be less than the third variance threshold. Additionally oralternatively to one or more of the examples disclosed above, the firstcondition can correspond to a prediction that the device is not securedto the object and the second condition can correspond to a predictionthat the device is secured to the object. Additionally or alternativelyto one or more of the examples disclosed above, determining that thedevice is not secured to the object when the predictions of thecondition of the device for the signal samples meets the threshold cancomprise predicting the first condition for a threshold number ofconsecutive predictions. Additionally or alternatively to one or more ofthe examples disclosed above, the object can be human skin. Additionallyor alternatively to one or more of the examples disclosed above, theobject can be a wrist.

Some examples of the disclosure are directed to non-transitory computerreadable storage medium. The computer readable medium can containinstructions that, when executed, perform a method for operating anelectronic device. The electronic device can include a processor. Themethod can comprise generating a plurality of signal samples, performingone or more variance calculations for each signal sample, predicting acondition of the device based on the one or more variance calculationsfor each signal sample, and determining that the device is not securedto the object when the predictions of the condition of the device forthe signal samples meets a threshold. Additionally or alternatively toone or more of the examples disclosed above, the sensor can comprise alight emitter and a light detector. Additionally or alternatively to oneor more of the examples disclosed above, the light emitter can a lightemitting diode (e.g., in the infrared range). Additionally oralternatively to one or more of the examples disclosed above, eachsignal sample can include a first signal generated when the lightemitter is on and a second signal generated when the light emitter isoff. Additionally or alternatively to one or more of the examplesdisclosed above, the first signal and second signal of a signal samplecan generated within threshold length of time (e.g., 250 microseconds).Additionally or alternatively to one or more of the examples disclosedabove, the one or more variance calculations can comprise a firstvariance calculation of a variance of a difference between the firstsignal and the second signal of the signal sample. Additionally oralternatively to one or more of the examples disclosed above, predictingthe condition of the device based on the one or more variancecalculations for each signal sample can comprise, predicting a firstcondition of the device when the first variance calculation is below afirst variance threshold and predicting a second condition of the devicewhen the first variance calculation meets or exceeds the first variancethreshold. Additionally or alternatively to one or more of the examplesdisclosed above, the one or more variance calculations can comprise asecond variance calculation of a difference between a variance of thefirst signal of the signal sample and a variance of the second signal ofthe signal sample. Additionally or alternatively to one or more of theexamples disclosed above, predicting the condition of the device basedon the one or more variance calculations for each signal sample cancomprise predicting a first condition of the device when the firstvariance calculation is below a first variance threshold; predicting thefirst condition of the device when the first variance calculation meetsor exceeds the first variance threshold, the first variance calculationis below a second variance threshold, and the second variancecalculation is below a third variance threshold; predicting a secondcondition of the device when the first variance calculation meets orexceeds the first variance threshold and the first variance calculationmeets or exceeds the second variance threshold; and predicting thesecond condition of the device when the first variance calculation meetsor exceeds the first variance threshold, the first variance calculationis below the second variance threshold, and the second variancecalculation meets or exceeds the third variance threshold. Additionallyor alternatively to one or more of the examples disclosed above, thesecond variance threshold can be greater than the first variancethreshold. Additionally or alternatively to one or more of the examplesdisclosed above, the third variance threshold can be less than the firstvariance threshold. Additionally or alternatively to one or more of theexamples disclosed above, predicting the condition of the device basedon the one or more variance calculations for each signal sample cancomprise predicting a first condition of the device when the firstvariance calculation is below a first variance threshold and the firstvariance calculation is below a third variance threshold; predicting thefirst condition of the device when the first variance calculation isbelow the first variance threshold, the first variance calculation meetsor exceeds the third variance threshold, and the second variancecalculation is below a fourth variance threshold; predicting a secondcondition of the device when the first variance calculation meets orexceeds the first variance threshold and the first variance calculationmeets or exceeds the second variance threshold; and predicting thesecond condition of the device when the first variance calculation isbelow the first variance threshold, the first variance calculation meetsor exceeds the third variance threshold, and the second variancecalculation meets or exceeds the fourth variance threshold. Additionallyor alternatively to one or more of the examples disclosed above, thesecond variance threshold can be greater than the first variancethreshold. Additionally or alternatively to one or more of the examplesdisclosed above, the third variance threshold can be less than the firstvariance threshold. Additionally or alternatively to one or more of theexamples disclosed above, the fourth variance threshold can be less thanthe third variance threshold. Additionally or alternatively to one ormore of the examples disclosed above, the first condition can correspondto a prediction that the device is not secured to the object and thesecond condition can correspond to a prediction that the device issecured to the object. Additionally or alternatively to one or more ofthe examples disclosed above, determining that the device is not securedto the object when the predictions of the condition of the device forthe signal samples meets the threshold can comprise predicting the firstcondition for a threshold number of consecutive predictions.Additionally or alternatively to one or more of the examples disclosedabove, the object can be human skin. Additionally or alternatively toone or more of the examples disclosed above, the object can be a wrist.

Although the disclosed examples have been fully described with referenceto the accompanying drawings, it is to be noted that various changes andmodifications will become apparent to those skilled in the art. Suchchanges and modifications are to be understood as being included withinthe scope of the disclosed examples as defined by the appended claims.

1. A device comprising: a sensor comprising a light emitter and a lightdetector configured to generate signal samples, wherein each signalsample includes a first signal generated when the light emitter is onand a second signal generated when the light emitter is off; andprocessing circuitry coupled to the sensor capable of: performing one ormore variance calculations for each signal sample, the one or morevariance calculations for each signal sample indicative of a firstcondition or a second condition; in accordance with a threshold numberof signal samples within a threshold period of time indicating the firstcondition, determining that the device is not secured to an object; andin accordance with fewer than the threshold number of signal sampleswithin the threshold period of time indicating the first condition,determining that the device is secured to the object.
 2. The device ofclaim 1, wherein the one or more variance calculations comprises a firstvariance calculation of a variance of a difference between the firstsignal and the second signal of the signal sample.
 3. The device ofclaim 2, wherein the one or more variance calculations for each signalsample indicate the first condition when the first variance calculationis below a first variance threshold and indicate the second conditionwhen the first variance calculation meets or exceeds the first variancethreshold.
 4. The device of claim 2, wherein the one or more variancecalculations comprises a second variance calculation of a differencebetween a variance of the first signal of the signal sample and avariance of the second signal of the signal sample.
 5. The device ofclaim 4, wherein the one or more variance calculations for each signalsample: indicate the first condition when the first variance calculationis below a first variance threshold; indicate the first condition whenthe first variance calculation meets or exceeds the first variancethreshold, the first variance calculation is below a second variancethreshold, and the second variance calculation is below a third variancethreshold; indicate the second condition when the first variancecalculation meets or exceeds the first variance threshold and the firstvariance calculation meets or exceeds the second variance threshold; andindicate the second condition when the first variance calculation meetsor exceeds the first variance threshold, the first variance calculationis below the second variance threshold, and the second variancecalculation meets or exceeds the third variance threshold.
 6. The deviceof claim 4, wherein the one or more variance calculations for eachsignal sample: indicate the first condition when the first variancecalculation is below a first variance threshold and the first variancecalculation is below a third variance threshold; indicate the firstcondition when the first variance calculation is below the firstvariance threshold, the first variance calculation meets or exceeds thethird variance threshold, and the second variance calculation is below afourth variance threshold; indicate the second condition when the firstvariance calculation meets or exceeds the first variance threshold andthe first variance calculation meets or exceeds the second variancethreshold; and indicate the second condition when the first variancecalculation is below the first variance threshold, the first variancecalculation meets or exceeds the third variance threshold, and thesecond variance calculation meets or exceeds the fourth variancethreshold.
 7. The device of claim 1, wherein the first conditioncorresponds to a prediction that the device is not secured to the objectand wherein the second condition corresponds to a prediction that thedevice is secured to the object.
 8. The device of claim 1, wherein thethreshold number of signal samples is equal to a number of signalsamples generated within the threshold period of time.
 9. A methodcomprising: generating a plurality of signal samples, wherein eachsignal sample includes a first signal generated when a light emitter ison and a second signal generated when the light emitter is off;performing one or more variance calculations for each signal sample, theone or more variance calculations for each signal sample indicative of afirst condition or a second condition; in accordance with a thresholdnumber of signal samples within a threshold period of time indicatingthe first condition, determining that the device is not secured to anobject; and in accordance with fewer than the threshold number of signalsamples within the threshold period of time indicating the firstcondition, determining that the device is secured to the object.
 10. Themethod of claim 9, wherein the one or more variance calculationscomprises a first variance calculation of a variance of a differencebetween the first signal and the second signal of the signal sample. 11.The method of claim 10, wherein the one or more variance calculationsfor each signal sample indicate the first condition when the firstvariance calculation is below a first variance threshold and indicatethe second condition when the first variance calculation meets orexceeds the first variance threshold.
 12. The method of claim 10,wherein the one or more variance calculations comprises a secondvariance calculation of a difference between a variance of the firstsignal of the signal sample and a variance of the second signal of thesignal sample.
 13. A non-transitory computer readable storage medium,the computer readable medium containing instructions that, when executedby an electronic device including a processor and a sensor comprising alight emitter and a light detector, performs a method comprising:generating a plurality of signal samples, wherein each signal sampleincludes a first signal generated when the light emitter is on and asecond signal generated when the light emitter is off; performing one ormore variance calculations for each signal sample, the one or morevariance calculations for each signal sample indicative of a firstcondition or a second condition; in accordance with a threshold numberof signal samples within a threshold period of time indicating the firstcondition, determining that the device is not secured to an object; andin accordance with fewer than the threshold number of signal sampleswithin the threshold period of time indicating the first condition,determining that the device is secured to the object.
 14. Thenon-transitory computer readable storage medium of claim 13, wherein theone or more variance calculations comprises a first variance calculationof a variance of a difference between the first signal and the secondsignal of the signal sample.
 15. The non-transitory computer readablestorage medium of claim 14, wherein the one or more variancecalculations for each signal sample indicate the first condition whenthe first variance calculation is below a first variance threshold andindicate the second condition when the first variance calculation meetsor exceeds the first variance threshold.
 16. The non-transitory computerreadable storage medium of claim 14, wherein the one or more variancecalculations comprises a second variance calculation of a differencebetween a variance of the first signal of the signal sample and avariance of the second signal of the signal sample.
 17. Thenon-transitory computer readable storage medium of claim 16, wherein theone or more variance calculations for each signal sample: indicate thefirst condition when the first variance calculation is below a firstvariance threshold; indicate the first condition when the first variancecalculation meets or exceeds the first variance threshold, the firstvariance calculation is below a second variance threshold, and thesecond variance calculation is below a third variance threshold;indicate the second condition when the first variance calculation meetsor exceeds the first variance threshold and the first variancecalculation meets or exceeds the second variance threshold; and indicatethe second condition when the first variance calculation meets orexceeds the first variance threshold, the first variance calculation isbelow the second variance threshold, and the second variance calculationmeets or exceeds the third variance threshold.
 18. The non-transitorycomputer readable storage medium of claim 16, wherein the one or morevariance calculations for each signal sample: indicate the firstcondition when the first variance calculation is below a first variancethreshold and the first variance calculation is below a third variancethreshold; indicate the first condition when the first variancecalculation is below the first variance threshold, the first variancecalculation meets or exceeds the third variance threshold, and thesecond variance calculation is below a fourth variance threshold;indicate the second condition when the first variance calculation meetsor exceeds the first variance threshold and the first variancecalculation meets or exceeds the second variance threshold; and indicatethe second condition when the first variance calculation is below thefirst variance threshold, the first variance calculation meets orexceeds the third variance threshold, and the second variancecalculation meets or exceeds the fourth variance threshold.
 19. Thenon-transitory computer readable storage medium of claim 13, wherein thefirst condition corresponds to a prediction that the device is notsecured to the object and wherein the second condition corresponds to aprediction that the device is secured to the object.
 20. Thenon-transitory computer readable storage medium of claim 13, wherein thethreshold number of signal samples is equal to a number of signalsamples generated within the threshold period of time.